- Email: [email protected]
High throughput synthesis and screening for functional materials
Applied Surface Science 223 (2004) 54–61
High throughput synthesis and screening for functional materials Xiao-Dong Xiang* Intematix Corp., 351 Rheem Boulevard, Moraga, CA 94556, USA
Abstract Synthesis and characterization tools designed for combinatorial materials science are described. The effectiveness of these tools is demonstrated through a well-known phase diagram study. An application of the technology to industrial product development is also described. # 2003 Published by Elsevier B.V. Keywords: Combinatorial material science; Ion-beam sputtering; Molecular beam epitaxy; Scanning micro-beam; X-ray diffraction; Fluorescence; Crystal structural; Composition mapping; Scanning evanescent microwave probe; Electrical impedance mapping; Continuous phase diagram mapping; Tunable dielectrics; Tunable dielectric filters
In the past 2 years, Intematix has developed a complete line of high throughput synthesis and screening tools for combinatorial materials science. The IBSD-200 (Fig. 1) is a versatile, UHV, multi-target ion beam sputtering deposition system designed for ex situ and in situ growth of continuous binary and ternary phase diagrams combinatorial synthesis and a variety of other thin film deposition applications. Similar to pulsed laser ablation (PLD), the energy beam is separated from targets, which makes switching target very easy. However, in contrast to PLD, which generate large particulates during deposition, ion beam deposition has been known to produce atomically smooth surface morphology even at low temperatures enabling multi-layer optical filter fabrication. In addition, ion beam can sputter either metal or non-metal compounds, such as oxides, nitrides, fluorides, or silicates, etc while PLD cannot deposit metallic and many nonabsorbing precursor targets (such as SiO2). * Tel.: þ1-925-631-9005; fax: þ1-925-631-7892. E-mail address: [email protected] (X.-D. Xiang).
0169-4332/$ – see front matter # 2003 Published by Elsevier B.V. doi:10.1016/S0169-4332(03)00931-0
The chamber is maintained at ultra-high vacuum of better than 109 Torr to prevent oxidation of deposited films. The system consists of an automated fivemetal target carousel and an x–y precision shutter (Fig. 2). A load–lock chamber is used for sample exchange without breaking the vacuum of the main chamber. The system is perfect for synthesis of continuous phase diagrams (CPDs) and other type of libraries from pure metal precursors without oxygen contamination. The IBSD-100 can also be configured for multi-layer device fabrication, ideal for materials R&D and pilot line production of thin film devices. Combinatorial MBE (CMBE) system (Fig. 3) is another synthesis tool that Intematix developed recently. The CMBE system is equipped with three Knudsen cells (with one cell capable of operating up to 1800 8C) and one e-beam gun. With the high temperature Knudsen cell and e-beam evaporator, the metals with low evaporation pressure, such as transition metals, can be deposited. The main chamber can be pumped by Cryo-pump and Ion pump to ultra-high
X.-D. Xiang / Applied Surface Science 223 (2004) 54–61
Fig. 1. Combinatorial ion beam sputtering deposition system.
Fig. 2. Schematics of automated target carousel and x–y precision shutter configuration of IBSD system.
55
56
X.-D. Xiang / Applied Surface Science 223 (2004) 54–61
Fig. 3. Combinatorial Molecular Beam Epitaxy (CMBE) system.
vacuum of 1010 Torr after baking. A rotatable substrate on a X–Y–Z manipulator can be heated up to 1000 8C even under reactive atmosphere (such as O2), which makes the system suitable for both metal alloy and oxides film growth. CMBE is complementary to the CIBS system, especially suitable for in situ ultrahigh quality epitaxial film growth of CPDs involving volatile precursors.
After the material chips are fabricated, detailed composition and structural characterization are desired for quality control and structure–physical property relationship studies. Intematix recently developed a scanning micro-beam X-ray with diffraction and X-ray fluorescence capabilities (Fig. 4), which meet both composition and structural characterization requirements very well. The system is able to map the
Fig. 4. Scanning micro-beam X-ray apparatus with diffraction and X-ray fluorescence capabilities.
X.-D. Xiang / Applied Surface Science 223 (2004) 54–61
57
Fig. 5. Ni–Fe–Co ternary phase mapping by CPD and its comparison to bulk alloy and nanoparticle structure phase diagrams.
crystal structural and compositional information of synthesized combinatorial materials chips with a spatial resolution in a range of 10–100 mm. This equipment includes a 5 kW X-ray source, a unique high brilliance X-ray micro-focus source designed for micro-diffraction measurement and an X-ray fluorescence spectrometer for simultaneous mapping of compositions. The focused flux intensity is at least 10–20 times higher than pinhole configuration with a same X-ray source. The sample can be scanned by a 1 in: 1 in. mechanical scanning stage. The facility includes associated electronics, motion control, a computer and software. Example of Fe–Ni–Co structural ternary phase diagram mapped by Intematix’s scanning micro-beam XRD is shown in Fig. 5. The thin film metal alloy ternary phase diagram was fabricated with CIBD
system with post-annealing process to study annealing rate effect. The sample was made in an equilateral triangle shape, and the elemental concentration varies from 0 to 100% for each two elements along each triangle edge. An X-ray diffraction pattern (y–2y) was characterized by fitting the peaks with a Gaussian curve. The 2y scan range of the diffraction pattern is 48, which is sufficient to cover the angular range of the characteristic diffraction peaks of Fe, Co, Ni, and some well-known compound phases. Fig. 5 shows simplified the phase boundaries mapped with our CPD basically agree with bulk sample phase diagram. Fe–Ni phase strip on one side of the ternary triangle phase diagram was selected to measure phase transformation shown in Fig. 6. The colored map indicates the relationship of the diffraction peak position, width
Fig. 6. XRD measurement of Fe–Ni phase strip: (a) annealed at 600 8C and cooled in a speed of 5 8C/min; (b) annealed at 600 8C and cooled in a speed of 0.5 8C/min.
58
X.-D. Xiang / Applied Surface Science 223 (2004) 54–61
Fig. 7. Photo of EMP and schematics of the probe design and monopoly filed.
and intensity versus the ratio of Fe–Ni. The Fe–Co–Ni amorphous phase spread was fabricated at room temperature. It was then annealed at 600 8C for homogeneous diffusion and phase formation. Fig. 6a shows the structural scan after it was cooled to room temperature in a rate of 58 min1. Fig. 6b shows the structural scan of the same sample after being heated to 600 8C again and cooled in a slow speed of 0.5 8C/min. Three different phases can be clearly identified in the colored map of Fig. 6a. In low Ni concentration region, the alloy has higher austenite transformation temperature > 600 8C. It keeps a-Fe structure (BCC), with a 2y value about 44.58 closely matching with (1 1 0) of BCC-Fe (2y ¼ 44:678), during and after annealing. As Ni content increases, the austenite (FCC) formed during annealing at 600 8C due to the gradually decease of the austenite forming temperature. In the range of 20–50% of Ni content, after cooling to room temperature at the rate of 5 8C/min, the austenite phase transformed into a martensite phase since Ms is higher than room temperature and the cooling rate is fast enough for non-equilibrium martensite to form. The volume expansion of ferrous martensite decreases the diffraction angle by about 1.08. With Ni content higher than 50%, the Ms become lower than room temperature. Consequently, the austenite (FCC) structure remains after cooling. It was kept as super-cooled austenite, but not decomposed into Ni and FeNi3 solid solutions due to very low phase diffusion speed at the temperature lower than 600 8C. With the increase of Fe content, the volume of the lattice of the austenite gradually increases causing the
Fig. 8. Local dielectric constants of different materials measured by EMP.
2y gradually decreases. In sharp contrast, Fig. 6b indicates that martensite phase almost disappears as the result of very slow cooling rate. The solid was kept as austenite state with a small portion of transformation into a-Fe solid solution. It is obvious that this cooling rate is slower than the critical cooling speed for martensite phase formation (martensite phase in most cases is a non-equilibrium phase). We expect the martensite phase disappears entirely as we further slows down the cooling rate. Another powerful high throughput screening tool developed at Intematix is the scanning evanescent microwave probe (SEMP)1 for electrical impedance mapping as shown in Fig 7a. SEMP is based on a high 1
Nondestructive imaging of dielectric constant profiles and ferroelectric domains with a scanning tip microwave near-field microscope [1].
X.-D. Xiang / Applied Surface Science 223 (2004) 54–61
59
Fig. 9.
quality factor (Q) microwave coaxial resonator with a sharpened metal tip mounted on the center conductor and extended beyond an aperture formed by a thin metal shielded end-wall of the resonator (Fig. 7b). The tip and the shielding structure are designed so that the propagating far-field components are shielded within the cavity, whereas the non-propagating evanescent waves are generated at the tip. This feature is crucial for both high resolution and quantitative analysis. If both evanescent and large component of propagating (leaked from the resonator) waves must be considered and calculated (as in all other types of microwave probes), the quantitative microscopy would be very difficult to achieve. The SEMP measures the interaction between the evanescent electromagnetic wave
(generated by a metal tip coupled to a microwave source) and the sample. Only when the tip is in a close range of the sample will the evanescent waves on the tip interact with the materials. The interaction gives a rise to resonant frequency and Q changes of the cavity Table 1 Materials
e
Tunability (4 V/mm)
Q (1 GHz)
FOM ((tunability Q)/E)
ST1200 ST900 ST400 ST100 ST80
1250 876 421 103 80
30% 20% 13% 11% (2 V/mm) 7.2% (1.8 V/mm)
662 1137 1600 729 1110
49 57 53 40 44
60
X.-D. Xiang / Applied Surface Science 223 (2004) 54–61
Fig. 10. High throughput screening of dielectric constant and tangent loss of a phase strip.
Table 2
Center frequency (MHz) Tunable range and percentage Applied voltage Qu Relative 3 dB bandwidth QL with IL ¼ 20 dB Relative 3 dB bandwidth QL with IL ¼ 3 dB Relative 3 dB bandwidth
High band
Low band
1710–2170 80 MHz or 4% Handset battery compatible 770 2.7 MHz 700 3 MHz 260 8 MHz
824–960 35 MHz or 4% Handset battery compatible 600 1.5 MHz 550 1.7 MHz 200 4 MHz
and consequently the microscopy of the electrical impedance. The quantitative analysis methods have been developed and proven capable of quantitative determination of local dielectric (and nonlinear dielectric) constant (Fig. 8) and microwave loss tangent.2 Since the measured signal is strongly dependent on tip–sample distance, an independent tip–sample distance control is desirable. Fig. 9 illustrates the tip– sample approaching curves of EMP signals and the signal readout by a force sensor integrated with the probe tip (Fig. 9a). The results illustrate that force sensor can be used as universal regulator to control tip–sample distance. Fig. 9b shows the scan images obtained by a force sensor enabled tip–sample feedback control mechanism. The above examples demonstrate that these technologies provide us very effective and reliable means 2 Quantitative microwave near-field microscopy of dielectric properties [2].
to carry out systematic materials science research with a much accelerated pace than conventional approaches. Recently, Intematix has successfully applied these tools to the development of industrial products. The first example is the development of advanced tunable materials for microwave tunable filters and other tunable devices used in portable wireless devices. High throughput screening tools are first used to identify candidate compositions with high performance characteristics (figure of merit defined as (tunability Q)/ electrical field required in a unit of V/mm) as exemplified in Fig. 10. After the candidates are identified, ceramic processes are used to optimize and validate the results. Table 1 list the properties of some of the best tunable dielectrics with figure of merits 5–10 times better than the state-of-art tunable dielectrics reported before. Development of function materials often does not stop at this stage. Usually, the materials have to be configured and tested in a real device format before it
X.-D. Xiang / Applied Surface Science 223 (2004) 54–61
becomes useful. Intematix has successfully designed special filter configuration tailored for these new materials and performed device simulation studies. Table 2 lists simulated parameters of a filter designed based on our new materials for an industrial application. This kind of performance has not been possible to achieve previously.
61
References [1] Y. Lu, T. Wei, F. Duewer, Y. Lu, N. Ming, P.G. Schultz, X.-D. Xiang, Nondestructive imaging of dielectric constant profiles and ferroelectric domains with a scanning tip microwave nearfield microscope, Sci. 276 (1997) 2004. [2] C. Gao, X.-D. Xiang, Rev. Sci. Instrum. 69 (1998) 3846.
High throughput synthesis and screening for functional materials Xiao-Dong Xiang* Intematix Corp., 351 Rheem Boulevard, Moraga, CA 94556, USA
Abstract Synthesis and characterization tools designed for combinatorial materials science are described. The effectiveness of these tools is demonstrated through a well-known phase diagram study. An application of the technology to industrial product development is also described. # 2003 Published by Elsevier B.V. Keywords: Combinatorial material science; Ion-beam sputtering; Molecular beam epitaxy; Scanning micro-beam; X-ray diffraction; Fluorescence; Crystal structural; Composition mapping; Scanning evanescent microwave probe; Electrical impedance mapping; Continuous phase diagram mapping; Tunable dielectrics; Tunable dielectric filters
In the past 2 years, Intematix has developed a complete line of high throughput synthesis and screening tools for combinatorial materials science. The IBSD-200 (Fig. 1) is a versatile, UHV, multi-target ion beam sputtering deposition system designed for ex situ and in situ growth of continuous binary and ternary phase diagrams combinatorial synthesis and a variety of other thin film deposition applications. Similar to pulsed laser ablation (PLD), the energy beam is separated from targets, which makes switching target very easy. However, in contrast to PLD, which generate large particulates during deposition, ion beam deposition has been known to produce atomically smooth surface morphology even at low temperatures enabling multi-layer optical filter fabrication. In addition, ion beam can sputter either metal or non-metal compounds, such as oxides, nitrides, fluorides, or silicates, etc while PLD cannot deposit metallic and many nonabsorbing precursor targets (such as SiO2). * Tel.: þ1-925-631-9005; fax: þ1-925-631-7892. E-mail address: [email protected] (X.-D. Xiang).
0169-4332/$ – see front matter # 2003 Published by Elsevier B.V. doi:10.1016/S0169-4332(03)00931-0
The chamber is maintained at ultra-high vacuum of better than 109 Torr to prevent oxidation of deposited films. The system consists of an automated fivemetal target carousel and an x–y precision shutter (Fig. 2). A load–lock chamber is used for sample exchange without breaking the vacuum of the main chamber. The system is perfect for synthesis of continuous phase diagrams (CPDs) and other type of libraries from pure metal precursors without oxygen contamination. The IBSD-100 can also be configured for multi-layer device fabrication, ideal for materials R&D and pilot line production of thin film devices. Combinatorial MBE (CMBE) system (Fig. 3) is another synthesis tool that Intematix developed recently. The CMBE system is equipped with three Knudsen cells (with one cell capable of operating up to 1800 8C) and one e-beam gun. With the high temperature Knudsen cell and e-beam evaporator, the metals with low evaporation pressure, such as transition metals, can be deposited. The main chamber can be pumped by Cryo-pump and Ion pump to ultra-high
X.-D. Xiang / Applied Surface Science 223 (2004) 54–61
Fig. 1. Combinatorial ion beam sputtering deposition system.
Fig. 2. Schematics of automated target carousel and x–y precision shutter configuration of IBSD system.
55
56
X.-D. Xiang / Applied Surface Science 223 (2004) 54–61
Fig. 3. Combinatorial Molecular Beam Epitaxy (CMBE) system.
vacuum of 1010 Torr after baking. A rotatable substrate on a X–Y–Z manipulator can be heated up to 1000 8C even under reactive atmosphere (such as O2), which makes the system suitable for both metal alloy and oxides film growth. CMBE is complementary to the CIBS system, especially suitable for in situ ultrahigh quality epitaxial film growth of CPDs involving volatile precursors.
After the material chips are fabricated, detailed composition and structural characterization are desired for quality control and structure–physical property relationship studies. Intematix recently developed a scanning micro-beam X-ray with diffraction and X-ray fluorescence capabilities (Fig. 4), which meet both composition and structural characterization requirements very well. The system is able to map the
Fig. 4. Scanning micro-beam X-ray apparatus with diffraction and X-ray fluorescence capabilities.
X.-D. Xiang / Applied Surface Science 223 (2004) 54–61
57
Fig. 5. Ni–Fe–Co ternary phase mapping by CPD and its comparison to bulk alloy and nanoparticle structure phase diagrams.
crystal structural and compositional information of synthesized combinatorial materials chips with a spatial resolution in a range of 10–100 mm. This equipment includes a 5 kW X-ray source, a unique high brilliance X-ray micro-focus source designed for micro-diffraction measurement and an X-ray fluorescence spectrometer for simultaneous mapping of compositions. The focused flux intensity is at least 10–20 times higher than pinhole configuration with a same X-ray source. The sample can be scanned by a 1 in: 1 in. mechanical scanning stage. The facility includes associated electronics, motion control, a computer and software. Example of Fe–Ni–Co structural ternary phase diagram mapped by Intematix’s scanning micro-beam XRD is shown in Fig. 5. The thin film metal alloy ternary phase diagram was fabricated with CIBD
system with post-annealing process to study annealing rate effect. The sample was made in an equilateral triangle shape, and the elemental concentration varies from 0 to 100% for each two elements along each triangle edge. An X-ray diffraction pattern (y–2y) was characterized by fitting the peaks with a Gaussian curve. The 2y scan range of the diffraction pattern is 48, which is sufficient to cover the angular range of the characteristic diffraction peaks of Fe, Co, Ni, and some well-known compound phases. Fig. 5 shows simplified the phase boundaries mapped with our CPD basically agree with bulk sample phase diagram. Fe–Ni phase strip on one side of the ternary triangle phase diagram was selected to measure phase transformation shown in Fig. 6. The colored map indicates the relationship of the diffraction peak position, width
Fig. 6. XRD measurement of Fe–Ni phase strip: (a) annealed at 600 8C and cooled in a speed of 5 8C/min; (b) annealed at 600 8C and cooled in a speed of 0.5 8C/min.
58
X.-D. Xiang / Applied Surface Science 223 (2004) 54–61
Fig. 7. Photo of EMP and schematics of the probe design and monopoly filed.
and intensity versus the ratio of Fe–Ni. The Fe–Co–Ni amorphous phase spread was fabricated at room temperature. It was then annealed at 600 8C for homogeneous diffusion and phase formation. Fig. 6a shows the structural scan after it was cooled to room temperature in a rate of 58 min1. Fig. 6b shows the structural scan of the same sample after being heated to 600 8C again and cooled in a slow speed of 0.5 8C/min. Three different phases can be clearly identified in the colored map of Fig. 6a. In low Ni concentration region, the alloy has higher austenite transformation temperature > 600 8C. It keeps a-Fe structure (BCC), with a 2y value about 44.58 closely matching with (1 1 0) of BCC-Fe (2y ¼ 44:678), during and after annealing. As Ni content increases, the austenite (FCC) formed during annealing at 600 8C due to the gradually decease of the austenite forming temperature. In the range of 20–50% of Ni content, after cooling to room temperature at the rate of 5 8C/min, the austenite phase transformed into a martensite phase since Ms is higher than room temperature and the cooling rate is fast enough for non-equilibrium martensite to form. The volume expansion of ferrous martensite decreases the diffraction angle by about 1.08. With Ni content higher than 50%, the Ms become lower than room temperature. Consequently, the austenite (FCC) structure remains after cooling. It was kept as super-cooled austenite, but not decomposed into Ni and FeNi3 solid solutions due to very low phase diffusion speed at the temperature lower than 600 8C. With the increase of Fe content, the volume of the lattice of the austenite gradually increases causing the
Fig. 8. Local dielectric constants of different materials measured by EMP.
2y gradually decreases. In sharp contrast, Fig. 6b indicates that martensite phase almost disappears as the result of very slow cooling rate. The solid was kept as austenite state with a small portion of transformation into a-Fe solid solution. It is obvious that this cooling rate is slower than the critical cooling speed for martensite phase formation (martensite phase in most cases is a non-equilibrium phase). We expect the martensite phase disappears entirely as we further slows down the cooling rate. Another powerful high throughput screening tool developed at Intematix is the scanning evanescent microwave probe (SEMP)1 for electrical impedance mapping as shown in Fig 7a. SEMP is based on a high 1
Nondestructive imaging of dielectric constant profiles and ferroelectric domains with a scanning tip microwave near-field microscope [1].
X.-D. Xiang / Applied Surface Science 223 (2004) 54–61
59
Fig. 9.
quality factor (Q) microwave coaxial resonator with a sharpened metal tip mounted on the center conductor and extended beyond an aperture formed by a thin metal shielded end-wall of the resonator (Fig. 7b). The tip and the shielding structure are designed so that the propagating far-field components are shielded within the cavity, whereas the non-propagating evanescent waves are generated at the tip. This feature is crucial for both high resolution and quantitative analysis. If both evanescent and large component of propagating (leaked from the resonator) waves must be considered and calculated (as in all other types of microwave probes), the quantitative microscopy would be very difficult to achieve. The SEMP measures the interaction between the evanescent electromagnetic wave
(generated by a metal tip coupled to a microwave source) and the sample. Only when the tip is in a close range of the sample will the evanescent waves on the tip interact with the materials. The interaction gives a rise to resonant frequency and Q changes of the cavity Table 1 Materials
e
Tunability (4 V/mm)
Q (1 GHz)
FOM ((tunability Q)/E)
ST1200 ST900 ST400 ST100 ST80
1250 876 421 103 80
30% 20% 13% 11% (2 V/mm) 7.2% (1.8 V/mm)
662 1137 1600 729 1110
49 57 53 40 44
60
X.-D. Xiang / Applied Surface Science 223 (2004) 54–61
Fig. 10. High throughput screening of dielectric constant and tangent loss of a phase strip.
Table 2
Center frequency (MHz) Tunable range and percentage Applied voltage Qu Relative 3 dB bandwidth QL with IL ¼ 20 dB Relative 3 dB bandwidth QL with IL ¼ 3 dB Relative 3 dB bandwidth
High band
Low band
1710–2170 80 MHz or 4% Handset battery compatible 770 2.7 MHz 700 3 MHz 260 8 MHz
824–960 35 MHz or 4% Handset battery compatible 600 1.5 MHz 550 1.7 MHz 200 4 MHz
and consequently the microscopy of the electrical impedance. The quantitative analysis methods have been developed and proven capable of quantitative determination of local dielectric (and nonlinear dielectric) constant (Fig. 8) and microwave loss tangent.2 Since the measured signal is strongly dependent on tip–sample distance, an independent tip–sample distance control is desirable. Fig. 9 illustrates the tip– sample approaching curves of EMP signals and the signal readout by a force sensor integrated with the probe tip (Fig. 9a). The results illustrate that force sensor can be used as universal regulator to control tip–sample distance. Fig. 9b shows the scan images obtained by a force sensor enabled tip–sample feedback control mechanism. The above examples demonstrate that these technologies provide us very effective and reliable means 2 Quantitative microwave near-field microscopy of dielectric properties [2].
to carry out systematic materials science research with a much accelerated pace than conventional approaches. Recently, Intematix has successfully applied these tools to the development of industrial products. The first example is the development of advanced tunable materials for microwave tunable filters and other tunable devices used in portable wireless devices. High throughput screening tools are first used to identify candidate compositions with high performance characteristics (figure of merit defined as (tunability Q)/ electrical field required in a unit of V/mm) as exemplified in Fig. 10. After the candidates are identified, ceramic processes are used to optimize and validate the results. Table 1 list the properties of some of the best tunable dielectrics with figure of merits 5–10 times better than the state-of-art tunable dielectrics reported before. Development of function materials often does not stop at this stage. Usually, the materials have to be configured and tested in a real device format before it
X.-D. Xiang / Applied Surface Science 223 (2004) 54–61
becomes useful. Intematix has successfully designed special filter configuration tailored for these new materials and performed device simulation studies. Table 2 lists simulated parameters of a filter designed based on our new materials for an industrial application. This kind of performance has not been possible to achieve previously.
61
References [1] Y. Lu, T. Wei, F. Duewer, Y. Lu, N. Ming, P.G. Schultz, X.-D. Xiang, Nondestructive imaging of dielectric constant profiles and ferroelectric domains with a scanning tip microwave nearfield microscope, Sci. 276 (1997) 2004. [2] C. Gao, X.-D. Xiang, Rev. Sci. Instrum. 69 (1998) 3846.